Many endoscopic applications require an endoscope that is sufficiently flexible for allowing the endoscope to travel through the curved passageways inside the body. For decades, physicians have used endoscopes that rely on flexible image and illumination bundles with loose glass fibers. In a short rigid portion at the tip of such endoscopes is an objective lens connected to the image bundle. The length of this rigid tip is crucial for the flexibility and maneuverability of the endoscope inside the body. Various types of objective lenses for these flexible fiber endoscopes are known.
With the miniaturization of video chips, flexible endoscopes with a chip at the tip of the endoscope became possible and new requirements for objective lenses for video endoscopes emerged. In video endoscopes the image sensor portion is connected to other electronic elements inside the housing of the chip. This chip housing contributes significantly to the length of the rigid portion at the tip of the flexible video endoscope. To keep the overall length of the rigid portion of this tip short, extreme length constraints have arisen for the construction of the objective lens for such video endoscopes.
Making this task more difficult is that video chips require an objective lens with a wide field of view and an even performance over the full chip format. That is because flexible video endoscopes operate in very narrow body cavities where orientation within these narrow body cavities is difficult. As such, video endoscopes require a large field of view in order to overview a large portion of the body cavity. The required field of view on the object side can be up to 155°. Adding to the difficulty of designing video endoscopes is the variable brightness in body cavities and the differing colors of the walls of the various body cavities. Thus, the numerical aperture or so-called F-number of the objective lens of a video endoscope must be adapted for use under these varying conditions. For example, an objective lens should work up to an F-number of 5.6 with diffraction limited resolution over the whole field. For better illuminated body cavities the F-number can be reduced to achieve a larger depth of field.
For practical reasons the last surface of the objective lens needs a minimum distance to the video chip and in particular the video chip cover glass. Any surface imperfection close to the video chip can cause a spot to appear within the image area. To focus an image on the video chip, the objective lens has to be moved backward and forward relative to the video chip to find the best focus. The objective lens for a video endoscope also has to be aligned with the center of a photo sensitive area of the video chip. The center of the photo sensitive area of the chip is not aligned to any outer dimensions of the chip housing. Thus, the objective lens has to be aligned optically to the photo sensitive area and not to the video chip housing.